Are there better times to make a reservation for? Can you note the request on your reservation? Or is it just a matter of luck?
We ate there in December for dinner and just asked when we checked in if we could sit by the tank. Our reservation was around 5pm I think. They seated us fairly soon, and we did get to sit right by the tank.
Our reservation in December was for 11:30, which is when they start seating. We were there earlier, and they lead us straight to a table by the aquarium. Honestly, we had never even asked to sit near the aquarium any of the times we have eaten there. I imagine that asking and being willing to wait for a table would go a long way to getting a table there.
Check to see when Disney DiveQuest is scheduled if you want extra entertainment. Add about an hour to the time listed. Then you might see goofy divers like me joining you from the other side of the glass.